Field of the Invention
The present invention relates to carbon molecular sieve membranes and gas separations utilizing the same.
Related Art
Membranes are often preferred to other gas separation techniques in industry due to the following advantages. The energy consumption for membranes is low as they do not require a phase change for separation. Membrane modules are compact, thereby reducing their footprint and capital cost. Membranes are also mechanically robust and reliable because they have no moving parts.
Polymer membranes in particular are used in a wide variety of industrial applications. They enable the production of enriched nitrogen from air. They separate hydrogen from other gases in refineries. They are also used to remove carbon dioxide from natural gas.
However, owing to the manufacturing processes and material structure, today's polymeric membranes cannot reach both high selectivities and permeabilities, because a trade-off exists between permeability and selectivity. Robeson formulated semi-empirical upper-bound trade-off lines for several gas pairs. (Robeson, “The upper bound revisited”, Journal of Membrane Science 2008, vol 320, pp 390-400 (2008)). Carbon membranes exceed this upper-bound and therefore are quite promising.
Since the production of crack-free, hollow fiber, carbon molecular sieve membranes (CMS membranes) in the late 80s, researchers have shown that these carbon membranes offer several advantages over polymeric membranes. They have better intrinsic properties and exhibit better thermal and chemical stability. Thus, they have minimal plasticization affects.
CMS membranes are produced by pyrolyzing polymeric precursor membranes (i.e., green membranes) at temperatures of about 400-700° C. in a controlled atmosphere. In regards to controlled atmosphere, US 2011100211A discloses the importance of oxygen doping during pyrolysis process. It claims that oxygen doping can be tuned in order to obtain the desired properties for the CMS membrane.
The properties of CMS membranes also depend upon the choice of precursor polymer. Various polymer precursors are disclosed in the non-patent as being suitable for formation of CMS membranes. U.S. Pat. No. 6,565,631 discloses the use of Matrimid and 6FDA/BPDA-DAM. U.S. Pat. No. 7,947,114 discloses the use of cellulose acetate polymer. US 2010/0212503 discloses the use of polyphenylene oxide (PPO).
While the above disclosures have shown that the CMS membrane materials have superior intrinsic characteristics compared to those of precursor polymeric materials, there still exists a challenge of making high flux CMS hollow fiber membranes. This challenge is related to the fiber substructure morphology. In hollow fiber membrane spinning, a composition including polymer and solvent (aka the dope solution) and a bore fluid are extruded from a spinneret. The bore is extruded from a circular conduit while the dope solution is extruded from an annulus directly surrounding the bore fluid.
The dope solution composition can be described in terms of a ternary phase diagram as shown in FIG. 1. The polymer loading and amounts of solvent and non-solvent are carefully controlled in order to produce a single phase that is close to binodal. That way, as the extruded bore fluid and dope solution exit the spinneret and traverse through an air gap, solvent evaporating from the dope solution causes the exterior of the dope solution to solidify, thereby forming an ultrathin, dense skin layer. As the nascent fiber is plunged into a coagulant bath containing non-solvent, exchange of solvent and non-solvent from the fiber to the bath and vice-versa causes the remaining, inner portion of the now-solidifying fiber to form a two-phase sub-structure of solid polymer and liquid solvent/non-solvent.
After drying to remove remaining amounts of the solvent and non-solvent, the spaces in the sub-structure formerly containing solvent and non-solvent are left as an interconnecting network of pores within that sub-structure that contribute towards high flux. The final result is an asymmetric green fiber comprising a thin, dense skin over a thick, less dense, porous sub-structure.
During pyrolysis process, the pore network in the substructure collapses and densifies with the result of producing an effectively much thicker dense skin layer. Since flux is dense skin layer-dependent, a very thick dense skin can significantly decrease the flux exhibited by the CMS membrane. While the use of higher glass transition temperature (Tg) polymers in the dope solution may lower the relative degree of substructure pore collapse, suitably high fluxes are predicted to remain elusive without a solution to the foregoing problem.
Researchers have come up with two different approaches for making high flux CMS hollow fiber membranes.
One approach is to form a thin walled fiber. Since essentially the entire fiber wall collapses during pyrolysis to form an effectively much thicker dense skin layer, the obvious method of increasing the permeance of a hollow fiber membrane is to decrease its overall wall thickness. The drawback of this method is that, as fiber wall thickness is reduced, the strength of the resultant CMS membrane is compromised.
Therefore, it is an object of the invention to provide a CMS membrane (and method making the same) having a relatively high flux that exhibits a satisfactory degree of strength.
Another approach is to form silica structures within the CMS membrane. US 20130152793 discloses the immersion of precursor hollow fibers in vinyl-trimethoxy silane (VTMS) for about 1 day, withdrawing them from the VTMS, and allowing them to remain in an ambient air environment for about another day. After removal, the VTMS impregnated in the fiber reacts with moisture in the air to form a silica structure in the pores of the fiber substructure. This silica structure prevents those pores from collapsing during the subsequent pyrolysis. While this approach helps improve the CMS membrane flux, it does require an additional lengthy processing step (immersion within VTMS) above and beyond conventional techniques. An additional processing step creates a bottleneck to the overall production process that was not previously present with conventional techniques. An additional processing step also introduces another opportunity for poorly controlled variables to lead to non-uniform CMS membranes over time. An additional processing step also increases the footprint of the manufacturing process. Moreover, VTMS is a flammable liquid requiring careful handling. As a result of the foregoing issues, from a cost, complexity, throughput rate, manufacturing uniformity, manufacturing footprint, and safety point of view, the approach advocated by US 20130152793 is not fully satisfactory.
Therefore it is another object of the invention to provide a CMS membrane (and method of making the same) that does not require an additional processing step beyond conventional techniques, which is relatively less expensive, which is less complex, which does not pose a bottleneck to an overall throughput of manufacture, and which is relatively more safe than the solution proposed by US 20130152793.